1. Field of the Invention
This invention relates generally to dielectric/paraelectric thin film material compositions and methods of preparation; and more particularly to dielectric/paraelectric thin film material compositions and methods for use in electronically tunable microwave device applications.
2. Discussion of the Related Art
There is a need for the design and fabrication of dielectric/paraelectric thin film materials having enhanced dielectric and insulating properties which may be adjusted for a particular use. Presently, expensive current driven ferrites, are used in phased array antennas. Enhanced tunability with minimal dielectric loss are important for paraelectric thin film composition materials. Additionally, dielectric materials having high dielectric constant and low dielectric loss at microwave frequencies are needed to minimize losses in a microwave system.
Phase array antennas can steer transmitted or received signals either linearly or in two dimensions without mechanically oscillating the antenna. In contrast to mechanical scanning, electronic scanning is achieved by the technical concept of changing the path-length in the material. In magnetic materials (e.g. ferrities) the path-length is changed by applying a current; this requires higher power (voltage and amps). Phase array antennas are currently constructed using ferrite phase shifting elements. Due to the type of circuit requirements necessary to operate these antennas, they are costly, large, and heavy.
Ferroelectric/paraelectric materials are advantageous over ferrite materials as phase shifters, namely: ferroelectric/paraelectric materials have lower power requirements because ferroelectric materials are voltage driven while ferrites are current driven. Ferroelectric/paraelectric materials allow faster phase shifting or faster switching speed compared to ferromagnetic materials (ferrite phase shifters are slow to respond to control signals and cannot be used in applications where rapid beam scanning is required). Additionally, ferrites are non-reciprocal meaning that there must be a separate transmitter and receiver while ferroelectric/paraelectric technology uses a single element both as transmitter and receiver for all frequencies. Semiconductor device phase shifters are also being developed for phased array antennas and they have faster response speeds than ferrite technology, however, a major drawback is that they have very high losses at microwave and millimeter-wave frequencies. Another disadvantage of semiconductor phase shifters is that they have limited power-handling capability. MEMS (microelectromechanical systems) based phase shifters are also being developed for scanning antenna applications, however the high cost of device packaging and poor reliability due to stickation and the under-developed process science for MEMS fabrication, makes them non-ideal candidates for affordable, reliable antenna applications.
Bulk ferroelectric materials are currently being developed as a replacement for the more expensive current driven ferrites, which are currently used in phased array antennas. The bulk ceramic material, Ba1-xSrxTiO3, barium strontium titanate, (BST), is a promising material for electronically tunable device applications such as electronically tunable mixers, delay lines, filters, capacitors, oscillators, resonators, and phase shifters. The tunability of this material arises because it is possible to change its dielectric constant with application of an electric field. The tunable dielectric constant results in a change in the phase velocity in the device allowing it to be tuned in real time for a particular application. Bulk ceramic BST phase shifters, in a microstrip geometry, have been demonstrated at 5-10 GHz. However, the relatively high loss tangent of these materials, especially at microwave frequencies, have precluded their use in phase shifter applications. The dielectric properties of theses bulk materials have been improved, that is, the loss tangents were reduced to less than 0.006 at 10 GHz. This reduction in loss tangent was achieved by the addition of MgO to form BST/MgO bulk ceramic composites. Utilization of these BST/MgO materials as phase shifting elements in this bulk ceramic form is still quite limited due to the large voltages, on the order of xe2x89xa71000 V, needed to bias these bulk materials in a microstrip geometry.
In order to successfully employ BST based thin films in tunable device applications, such as phase shifters in phased array antennas, the dielectric and insulating properties should satisfy various material requirements. These requirements include: (1) a low loss tangent (tan xcex4) over the range of operating dc bias voltages, (2) a large variation in the dielectric constant with applied dc bias, e.g., a high tunability, (3) for impedance matching purposes, the dielectric constant (xcex5r) should be less than 500, and (4) the film should possess low leakage current (IL)/high film resistivity (xcfx81) characteristics. In addition to these primary material requirements, long-term reliability issues may also be considered. These reliability issues include: good stability of the dielectric and insulating properties over a broad range of frequency and temperature, and maximum reproducibility of the dielectric properties with respect to the applied dc bias, the film should be single phase with a dense microstructure and minimal defects, the films surface morphology should be smooth and crack free, and the film-substrate interface should be thermally stable as a function of processing temperature. It is desirable to have phase shifting materials with high tunability and minimum electronic loss (dissipation of microwave energy in the material).
Various thin film fabrication techniques have been attempted. Ba1-xSrxTiO3 thin films offer tunabilities upward of 50% at bias voltages of less than 10 V, which is compatible with the voltage requirements of present semiconductor based systems. Unfortunately, the tradeoff for such high tunabilities are high loss tangents, that is, tan xcex4 much larger than 0.03. Ferroelectric thin films are fabricated via a variety of film fabrication techniques, namely, pulsed laser deposition (PLD) technique, sputtering deposition technique, metalorganic chemical vapor deposition (MOCVD) technique. PLD offers a quick method of film disposition with precise control of the films stoichiometry. PLD offers poor film uniformity, and undesirable nano-particles are commonly incorporated into the condensed film. Thus, PLD is considered a small area (0.5xe2x80x3xc3x970.5xe2x80x3) deposition method useful only for xe2x80x9cscreening material compositionsxe2x80x9d and it is difficult, if not impossible, to scale-up this deposition method to useful wafer diameters, e.g., ranging from 4 inch to larger than 6 inch diameters. Additionally, PLD is not an xe2x80x9cindustry standardxe2x80x9d method for film fabrication used in the semiconductor industry.
Sputtering is an industry standard fabrication technique for semiconductor device fabrication. Although the sputtering technique is xe2x80x9cindustry standardxe2x80x9d it is difficult to achieve the desired stoichiometry control required for fabrication of Ba1-xSrxTiO3 based thin films. Metalorganic chemical vapor deposition (MOCVD) is another commonly utilized technique for fabrication of ferroelectric/paraelectric thin films which is considered xe2x80x9cindustry standardxe2x80x9d. However, the MOCVD growth method has difficulty maintaining film stoichiometry due to the possibility of reaction of the precursor elements in the vapor phase prior to deposition onto the substrate. Precise stoichiometric control of the film is necessary to obtain the desired film composition required for optimum/enhanced dielectric and electrical properties.